Birdcaging and the collapse of rods and cables in fixed-grip compression

D.M. Stump & G.H.M. van der Heijden

Wound cables and straight rods exhibit lateral instabilities when loaded
under compression and rotation in fixed-grip conditions. In a multi-strand
cable made from helically wound strands, this produces a `birdcage'
structure where the constituent strands separate to leave a central void
region. For a straight rod, a similar instability occurs when the planar
elastica becomes unstable under significant axial compression. The large
deflection theory of linear elastic rods is used to explain these
behaviours in terms of the standard concepts of the theory of buckling,
post-buckling, and imperfection sensitivity. The problem provides an
excellent vehicle for the use of Euler parameters (quaternions) which
remove the singularities normally associated with an Euler angle formulation.